Why do teachers make a big deal about PDA?

Me an my boyfriend were just standing close together our arms were on our bodies and not each other and the teacher tells us to stop making out. Then when my boyfriend leaves she says "thank you for escorting her to class I don't think she would've found it without you" in the most sarcastic way. No other teachers care about PDA. Not even the principle says anything to us? But we've got in trouble for hugging before. We're not one of those couples to make out in front of everyone we save that stuff for when were alone